The present disclosure relates to an image reader that detects reflected light from a manuscript.
In known art, an image reader is known that detects light reflected from a manuscript in order to convert the manuscript to image data. In general, when the image reader reads the manuscript, the image reader irradiates light onto a manuscript that is placed on a manuscript platen and detects light reflected back from the manuscript using a sensor that includes a plurality of image pickup devices. However, variations occur in output signals of each of the image pickup devices. In order to reduce the influence of these variations, calibration is performed in the image reader. For example, an image forming apparatus is known in which a reference white plate is provided on a top surface of an end portion of a manuscript platen that is formed of glass. The image forming apparatus reads the reference white plate before reading the manuscript placed on the manuscript platen and performs so-called shading correction (that is equivalent to calibration). When the reference white plate is provided on the top surface of the manuscript platen as in the image forming apparatus, an environment between the manuscript placed on the manuscript platen and a detecting element (a distance, a thickness of the manuscript platen and so on) is the same as an environment between the reference white plate and the detecting element. Thus, a focal point of the detecting element matches both the manuscript and the reference white plate. As a result, the image forming apparatus can perform the reading of the manuscript and the calibration in an appropriate manner.